Asthma and the Curse of Cleanliness

Hepatitis A, a liver-infecting virus commonly associated with poor sanitation, may be a guardian angel against asthma and allergies, says immunologist Dale Umetsu of Stanford University. This unexpected result may finally explain the enigmatic rise of these disorders in the United States.

Earlier studies have shown that the cleaner an area is, the more likely it is that children there suffer from asthma. Some researchers have proposed a “hygiene hypothesis” that sterile surroundings deprive infants’ immune systems of proper training. In search of the root cause, Umetsu and his colleagues focused on the possible role of hepatitis A, a nonfatal disease spread by contact with human feces.

Since the 1970s, the hepatitis A infection rate has dropped from nearly 100 percent to less than 30 percent; meanwhile, the incidence of asthma has doubled. After analyzing hundreds of DNA samples, Umetsu found a link. More than half of Americans carry a variant of an immune-response gene, TIM-1, that makes people much less likely to get asthma—but only if they have been exposed to hepatitis A.

“The statistics show that carrying this gene, combined with being infected with hepatitis A, conveys an enormous protection against asthma, allergies, and related diseases,” Umetsu says. He suspects that hepatitis A infection activates the TIM-1 immune receptor, which blocks the allergic and asthmatic response. Although the mechanism remains unknown, this finding begins to expose the biology behind the hygiene hypothesis.

It also seems to debunk theories blaming vaccines or antibiotics for the rise in asthma. “Hepatitis A infection rates are a good indicator of cleanliness, vaccination rates, and antibiotic use,” he says. “It seems we should be looking at hepatitis A, and possibly a few other specific diseases, rather than general indicators or infections as a whole.”